Two-pole circuit breakers used with individual loads or as the main of so-called "panels" (panelboards and load centers) commonly have a 5 or 10,000 ampere current-interruption rating at 240 volts AC. However, the available current from a single phase 240 volt AC source is often greatly in excess of 10,000 amperes, often five or ten times as much.
Fuses have recently come into use as the "main" protective device or interrupter for panels of residential-class circuit breakers energized by AC lines having high levels of available current. However, unlike circuit breakers, fuses have the usual disadvantages that they must be replaced when blown, and they cannot be operated like a switch to turn the power service on and off. This practice is illogical, but it has been adopted for lack of molded-case circuit breakers of reasonable proportions having high current-interrupting capacity. Conventional circuit breakers have been used for years as the main breaker of a panel even in installations where the available current exceeds the interruption capacity of the breaker; but in that case, a much larger circuit breaker upstream in the distribution system is actually relied on for clearing extreme faults in the panel.
A molded-case circuit breaker of the so-called quick-lag type is available having a 75,000 Amp interruption rating. That particular circuit breaker (in common with the usual molded-case circuit breakers rated at 5,000 or 10,000 Amp interrupting capacity) dependes for arc-interruption upon the AC voltage crossing zero after the incidence of a short-circuit. Moreover, the current through such circuit breakers tends to rise to approximately the same peak level that would occur were the breaker to remain closed after incidence of the fault. Consequently, a very high level of arcing energy develops in such breakers when interrupting short circuits. Relatively severe arcing damage to the contacts and in and near the arc chute results in such circuit breakers. Special high-strength molded cases are required. Seemingly, they depend upon electrodynamic effects in the particular configuration of the current path through the conductive parts identified with the contacts for promoting contact-opening operation. Such high values of electrodynamic contact-opening force is not available for short circuits substantially below the 75,000 Amp rating of such circuit breakers. Whether for this reason or becuase of the severe arcing damage that must occur during severe short circuits, these circuit breakers have apparently attained only limited acceptance.
The circuit breakers involved here are known as molded-case in the industry. The operating mechanism and the arc chambers are enclosed in cases of molded insulation. The arcs that develop after the contacts open under short circuit conditions are interrupted in the air of the arc chambers. Low voltage breakers of the so-called "air breaker" class are far too bulky and expensive for purposes of the present invention. Parenthetically, even such air breakers generally have inferior current-interruption capacities, compared to that which can be attained pursuant to the invention.